Command line arguments in C/C++

The most important function of C/C++ is main() function. It is mostly defined with a return type of int and without parameters :

int main() { /* ... */ } 

We can also give command-line arguments in C and C++. Command-line arguments are given after the name of the program in command-line shell of Operating Systems.
To pass command line arguments, we typically define main() with two arguments : first argument is the number of command line arguments and second is list of command-line arguments.

int main(int argc, char *argv[]) { /* ... */ }

or

int main(int argc, char **argv) { /* ... */ }

• argc (ARGument Count) is int and stores number of command-line arguments passed by the user including the name of the program. So if we pass a value to a program, value of argc would be 2 (one for argument and one for program name)
• The value of argc should be non negative.
• argv(ARGument Vector) is array of character pointers listing all the arguments.
• If argc is greater than zero,the array elements from argv[0] to argv[argc-1] will contain pointers to strings.
• Argv[0] is the name of the program , After that till argv[argc-1] every element is command -line arguments.

For better understanding run this code on your linux machine.

Input:

$ g++ mainreturn.cpp -o main 
$ ./main geeks for geeks

Output:

You have entered 4 arguments:
./main
geeks
for
geeks

Note : Other platform-dependent formats are also allowed by the C and C++ standards; for example, Unix (though not POSIX.1) and Microsoft Visual C++ have a third argument giving the program’s environment, otherwise accessible through getenv in stdlib.h: Refer C program to print environment variables for details.

Properties of Command Line Arguments:

1. They are passed to main() function.
2. They are parameters/arguments supplied to the program when it is invoked.
3. They are used to control program from outside instead of hard coding those values inside the code.
4. argv[argc] is a NULL pointer.
5. argv[0] holds the name of the program.
6. argv[1] points to the first command line argument and argv[n] points last argument.

Note : You pass all the command line arguments separated by a space, but if argument itself has a space then you can pass such arguments by putting them inside double quotes “” or single quotes ”.

Output in different scenarios:

1. Without argument: When the above code is compiled and executed without passing any argument, it produces following output.

   $ ./a.out
   Program Name Is: ./a.out
   No Extra Command Line Argument Passed Other Than Program Name
   

2. Three arguments : When the above code is compiled and executed with a three arguments, it produces the following output.

   $ ./a.out First Second Third
   Program Name Is: ./a.out
   Number Of Arguments Passed: 4
   ----Following Are The Command Line Arguments Passed----
   argv[0]: ./a.out
   argv[1]: First
   argv[2]: Second
   argv[3]: Third
   

3. Single Argument : When the above code is compiled and executed with a single argument separated by space but inside double quotes, it produces the following output.

   $ ./a.out "First Second Third"
   Program Name Is: ./a.out
   Number Of Arguments Passed: 2
   ----Following Are The Command Line Arguments Passed----
   argv[0]: ./a.out
   argv[1]: First Second Third
   

4. Single argument in quotes separated by space : When the above code is compiled and executed with a single argument separated by space but inside single quotes, it produces the following output.

   $ ./a.out 'First Second Third'
   Program Name Is: ./a.out
   Number Of Arguments Passed: 2
   ----Following Are The Command Line Arguments Passed----
   argv[0]: ./a.out
   argv[1]: First Second Third
   

References:
http://www.cprogramming.com/tutorial/lesson14.html
http://c0x.coding-guidelines.com/5.1.2.2.1.html